The firm sent out emails to all customers eligible for the promotion today. Anyone with an email address ending in 'ac.uk', 'ac.im', 'ac.gg', 'ac.je' or 'open.ac.uk' qualified for the offer that kicked off on 30 September and applies to upgrades of Windows Ultimate and Windows Professional.

One reader told us he followed Digital River’s instructions to the letter about how to download the software from its site.

“So I copy and paste the download link into Firefox. I wait... and the browser freezes... I try again. The same thing. I try again, this time on IE. This is the message I got: ‘Sorry, too many download attempts: 11. You were allowed to download only 10 times’,” he said.

"Each time I try I get the same message but the number of download attempts is now at 20. EPIC FAIL!! A maximum of 10 times????? I've even paid an extra £4 for 'Extended Download Service' and I can't download the f*cking thing!"

Academic staff and students have begun complaining about the cockup on forums and via Twitter, all of which seems to suggest this issue isn’t an isolated one.

We asked Digital River to explain what had gone wrong, but a worker at the company’s office in Maidenhead told us we’d have to wait to speak to someone at the company’s US headquarters about the download failure. Which struck us as odd given that the problem is clearly affecting customers on this side of the pond.

El Reg also asked Microsoft if it knew about the problem. At time of writing it hadn’t got back to us with comment. ®